1. Field of the Invention
This invention relates to a method for the early detection of potential blowout conditions during the drilling of a well. More particularly, this invention relates to such a method to determine when fluid from a subterranean formation enters the borehole of a well being drilled using a circulating drilling fluid.
2. Description of the Prior Art
In drilling wells it is common practice to circulate a drilling fluid past the drill bit to cool the bit and help carry cuttings out of the borehole to the surface of the well. It is an important consideration during drilling to keep the pressure exerted on the wellbore by the drilling fluid substantially balanced against the pressure of the formation being drilled into. If the pressure of the drilling fluid becomes considerably less than the pressure of the formation, there is danger that there will be a blowout with fluids from the formation blowing to the surface, disrupting circulation of the drilling fluid, interrupting drilling operations and sometimes resulting in loss of the well. The pressure to be expected in formations usually increases with depth in a manner which can be approximately calculated. However in many sections of the world abnormally pressured formations are encountered during drilling wherein the pressure in a strata is either sharply lower or sharply higher than would be expected. If the pressure of the formation fluid becomes greater than the pressure of the drilling fluid, formation fluid will tend to flow into the wellbore. Generally this formation fluid will have a lower density than the drilling fluid, especially if it consists of or contains a gas. Thus the imbalance between the pressure exerted by the drilling fluid, now contaminated with a relatively low density fluid, and that exerted by the formation fluid increases sharply and a blowout may result.
It is evident that it is desirable to know as soon as possible when formation fluids begin to enter the wellbore so that remedial action may be taken, such as increasing the pressure of the drilling fluid as by increasing its density. Various attempts to solve this problem have been proposed in the past. A flapper valve has been installed in a flow line to indicate changes in flow rate. Magnetic flow meters have been installed on both the standpipe and the flow line to detect differences between the in-going and the out-coming flow. Gauges have been installed on the mud pit to detect any changes in level of the mud pit. A pressure gauge has been placed at the surface in the line through which the drilling fluid is discharged from the well. None of these methods has proven to be entirely satisfactory.
It is an object of this invention to provide a method for detecting entry of formation fluids into the wellbore soon after the beginning of such entry.
It is another object to provide such a method to minimize danger of a blowout.
It is still another object to provide such a method which is operable even in wells containing a float or a back pressure valve in the tubing.
Other objects, advantages and features will be apparent from a consideration of the following description, drawings and claims.